1. Field of the Invention
The present invention relates to a method for producing a photovoltaic element with high conversion efficiency of energy from light to electricity, based on effective utilization of incident light.
2. Related Background Art
In search of a semiconductor layer for a photovoltaic element, research and development has extensively been carried out on materials including so-called thin film semiconductors using crystal-based semiconductors of monocrystalline or polycrystalline silicon with high conversion efficiency, amorphous silicon (hereinafter referred to as a-Si) available at low prices, and compound semiconductors such as CdS.multidot.CuInSe.sub.2. In any photovoltaic element, the technology leading to the more effective utilization of incident light is one of the most important technologies for removing more electric energy from a limited area.
This technology for effectively utilizing incident light involves techniques related to a transparent electrode for allowing more light to reach the semiconductor layer by preventing reflection on an incident surface of light, a light scattering layer for scattering the incident light to increase optical path lengths in the semiconductor layer, a reflective layer for reflecting light not absorbed by the semiconductor layer, again into the semiconductor layer to utilize the reflected light, and so on.
For example, U.S. Pat. Nos. 4,419,533 and 4,532,372 disclose technology for effectively utilizing the incident light by providing the surface of a reflective layer with a textured structure and scattering the light thereby. They also disclose the technology for forming a transparent, conductive layer on the reflective layer to prevent a short circuit due to defects of the semiconductor layer. The above-mentioned prior art discloses that aluminum, gold, silver, and copper can be used for the reflective layer. However, gold and silver are expensive and silver and copper are electrically ionized in the presence of even a little water, thereby increasing the likelihood of short circuit through the semiconductor layer.
As methods for forming a film of aluminum which is most suitable for the reflective layer among these metals, the following proposals were presented from a viewpoint of enhancing reliability by mainly controlling grain size.
Specifically, Japanese Patent Application Laid-Open No. 62-211377 discloses a method for forming the aluminum film by carrying out sputtering while supplying oxygen gas and controlling the flow rate of the oxygen gas while monitoring the amount thereof by a quadrupole mass spectrometer and describes that the grain size of aluminum can be controlled by the method.
Japanese Patent Application Laid-Open No. 2-297737 discloses a method for forming an aluminum film by sputtering wherein the sputter gas is kept under a pressure of not less than 10 mTorr, thereby preventing corrosion or a deformation defect thereof.
Japanese Patent Application Laid-Open No. 5-171434 discloses a method for forming an aluminum film having no projections in such a way that sputtering is carried out with residual air remaining in a vacuum vessel.
Japanese Patent Application Laid-Open No. 6-116723 discloses a method for obtaining a smooth aluminum film by repetitively carrying out a step of forming an aluminum film by sputtering and a step of exposing this aluminum film to a mixed gas of nitrogen and oxygen.
Japanese Patent Application Laid-Open No. 7-224379 discloses a method of forming an aluminum film by sputtering while introducing oxygen, hydrogen, or water, in addition to an inert gas.
The above-mentioned prior art disclosing the methods for forming an aluminum film discloses nothing about reflectivity of light, particularly, when used for the reflective layer of a photovoltaic element. The art also discloses nothing about the effect on an aluminum film when a transparent, conductive layer of zinc oxide or the like, and a semiconductor layer are stacked on the aluminum film. Further, the above-mentioned prior art describes nothing about change in quality of a formed aluminum film during a continuous film forming method for a long period of time.
The conversion efficiency of photovoltaic elements is expected to increase remarkably by the combination of a transparent electrode, a light scattering layer, and a reflective layer, whereby reduction of price of obtained electric energy is expected. In practice, however, it is often the case that the expected effect is not achieved, and the reduction of electric energy price thus far obtained is not satisfactory. In addition, achievement of long-term reliability throughout 10 to 20 years, as well as high conversion efficiency, is not yet satisfactory. Hence, photovoltaic elements are not yet in widespread system power use.
The aforementioned U.S. Pat. No. 4,419,533 and others disclose that aluminum, gold, silver, and copper can be used for the reflective layer for reflecting the light not absorbed by the semiconductor layer, again into the semiconductor layer to utilize the reflected light. The inventor has found that these metals have good reflectivities, but also have problems in other respects.
First, gold and silver are expensive. Further, when silver or copper is used, there is a problem that short circuit is likely to occur through the semiconductor layer, resulting in failure to extract power. This problem arises, particularly, when only a part of the photovoltaic element is irradiated with light in the presence of moisture for a long period.
After extensive and intensive study, the inventor has found the following facts. When the light irradiates only a part of the photovoltaic element, a potential opposite the polarity of a potential appearing in normal photoelectric conversion is applied to portions of the photovoltaic element which are not irradiated with the light. The action of this opposite potential and water not shut out by protection of an inexpensive resin ionizes the metal element of the reflective layer after long-term use, thus causing short circuit of the semiconductor layer. It has also been found that aluminum and gold are free of this problem.
The inventor has also found that the phenomenon of short circuit of the semiconductor layer can be prevented by sufficiently shutting the water out with provision of a protective film of a vitreous material or the like and that it is difficult to produce an inexpensive, lightweight, and flexible photovoltaic element by this method.
Based on the findings, the inventor has intensively and extensively studied a method for producing a reflective layer by use of aluminum, which is inexpensive and stable and has good reflectivity, or a material mainly containing aluminum, from various aspects.
Methods for producing the aluminum reflective layer on the substrate by sputtering are generally known, as described in U.S. Pat. No. 4,532,372 and others.
Also generally known are methods for stacking the transparent, conductive layer of zinc oxide or the like on the reflective layer or for stacking the semiconductor layer directly on the reflective layer, in order to prevent short circuit between the electrodes due to a pinhole or the like of the semiconductor layer. The inventor has, however, found that stacking of the aluminum layer with the transparent, conductive layer or the semiconductor layer may cause a problematic decrease in reflectivity, as compared with the structure of the single aluminum layer.
The inventor has also found that, especially when a method for continuously producing the photovoltaic element on a long substrate unwound from a substrate roll is employed as an inexpensive production method, there is a problematic decrease in reflectivity with the elapse of time during production for a long period. It has been determined that this is caused by the following: crystallization of the aluminum film proceeds due to energy upon deposition of the transparent, conductive layer or the semiconductor layer above the reflective layer, grain boundaries develop therewith, and the grain boundaries absorb the light to lower the reflectivity, thereby decreasing the quantity of light that can be converted to electricity in the semiconductor layer. The inventor has also determined the cause of the problem wherein reflectivity of the deposited film gradually decreases from start to end of formation in the continuous production method for a long term: a residual gas remaining in the vacuum system and containing water as the main component decreases with the passage of time, grain boundaries develop with increase in speed of crystallization of the aluminum film, and the grain boundaries absorb the light to lower the reflectivity, thereby decreasing the quantity of light that can be converted to electricity in the semiconductor layer.
It has been thus difficult heretofore to obtain a desired reflective layer with a good reflectivity, at a reasonable price, and with high reliability continuously for a long period by simple sputtering of aluminum by the conventionally known methods.